The present invention relates to an optical projection printing apparatus capable of aligning a mask pattern with a semiconductor wafer in high accuracy.
The optical projection printing apparatus of the prior art has a structure shown in FIG. 1a, as described on pp. 723 of IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-26, No. 4, April, 1979. In this apparatus, the light from a first light source (although not shown), i.e., the exposure light 8 passes through a condenser lens 30 into a mask 31. The light having transmitted through the mask 31 passes through projection lenses 32 and 33 to a wafer 34 to focus the image formed in the mask 31 on the wafer 34. The light source system thus constructed provides the so-called "Telecentric optical system" not only at the side of the object or mask 31 but also at the side of the image, i.e., the wafer 34, so that the main optical beam for the focusing is incident at a right angle upon the wafer 34 and the mask 31, as indicated by a broken line, at the sides of both the wafer 34 and the mask 31. For the light of alignment, a laser beam 35 is used as a second light source. This laser beam 35 is reflected by a small mirror 36 interposed between the projection lenses 33 and 32 and is incident in the grating pattern (i.e., the pattern, as designated at 4-1 in FIG. 1b) to be used as a wafer mark. Of the reflected lights from the grating pattern, only the .+-.1st order diffraction lights are selected by a special filter 37, which is interposed between the two projection lenses 32 and 33, and is focused on the mask 31. In one end portion of this mask 31, there is arranged the grating pattern as the mask pattern, which is aligned with the pattern from the wafer mark so that the misalignment between the wafer 34 and the mask 31 is detected by a later-described photodetector.
In order to raise the aligning accuracy, the following trial has been made in the prior art.
Designated at reference numeral 38-2 in FIG. 1a is a well-known Savart plate, by which double images are obtained. Double images composed of transversely shifted images of the wafer mark. And double images have perpendicularly polarized components each other. These double images are aligned with the mask mark. The result of a alignment is detected by a photo-detector (although not shown) through an optical element 38-1 which is composed of an electro-optical modulator and a detector. As is well known in the art, the optical element composed of the electro-optical modulator and the detector is enabled to timely alternately select the perpendicularly polarized components of the double images of the wafer mark by the voltage applied to the electro-optical modulator so that the double images of the wafer mark and the alignment signal of the mask mark can be timely separately obtained.
In the method described above, however, the transverse shift of the diffraction image have to be equal in high accuracy. For this high accuracy, the optical elements to be used have to be constructed in high accuracy and freed from any misalignment with time.
The prior art thus far described is limited in the following point and has a defect. If the wavelengths of the exposure light as the first light source and the aligning laser beam as the second light source are largely different, the correction of chromatic aberration of the projection lenses is not sufficient so that the light reflected from the aforementioned wafer mark cannot be focused on the mask 31. Thus, the prior art cannot be used.
In the optical system shown in FIG. 1a, the direction of the grating pitch of a wafer mark 4-1 is parallel with the direction of segment joining the point P intersection between the wafer 34 and an optical axis 100, and the wafer mark 4-1, as shown in FIG. 1b. In other words, in the prior art, as shown in FIG. 1b, the direction of the grating pitch of the wafer mark 4-1 composed of the grating pattern is contained in the so-called "meridional plane" 102, which is defined by the plane including the optical axis 100 of the first optical system shown in FIG. 1 and the position of the wafer mark.
Specifically, the position of the pattern, which is focused by the .+-.1st order diffraction lights on the mask 31, is located in the meridional plane (i.e., the plane of FIG. 1a) of the projection lenses shown in FIG. 1, that is, in the meridional plane of FIG. 1b. As a result, the center line (as indicated by the broken line in FIG. 1a) of the .+-.1st order diffraction lights, i.e., the main beam is not normal to the mask plane due to the chromatic aberration of the projection lenses. This causes a defect that the detected position by the aligning laser beam is misaligned.
Since, moreover, the filter for extracting the .+-.1st order diffraction lights from the wafer mark is interposed between the projection lenses 32 and 33, it may undesirably block a portion of the exposure light for transferring the mask pattern to the wafer.